The maker of the popular Firefox browser is moving ahead with plans to block the most common forms of Internet tracking, allowing hundreds of millions of users to limit who watches their movements across the Web, company officials said Wednesday.

The decision comes despite intense resistance from advertising groups, which have argued that tracking is essential to delivering well-targeted, lucrative ads that pay for many popular Internet services. When Firefox’s maker, Mozilla, first publicly suggested that it might limit blocking in February, one advertising executive called it “a nuclear first strike” against the industry.

Widespread release of the blocking technology remains months away, but Mozilla officials spoke confidently on Wednesday about the growing sophistication of tools they are building to limit the placement of “cookies” in the browsers of individual users.

These bits of code, often placed by data collection companies users have never heard of, allow the companies to learn what sites the browser visits for many months or even years. Tracking would still be allowed by Firefox if users gave a website express permission, or if users visited regularly — as is common with shopping, social media or news sites.

“We’re trying to change the dynamic so that trackers behave better,” said Brendan Eich, chief technology officer for Mozilla, a nonprofit group. Its Firefox browser is used by about 20 percent of the world’s desktop computers, according to NetMarketShare.

The blocking technology that Mozilla is developing borrows heavily from Apple’s Safari browser, which blocks all “third-party” cookies, meaning bit of tracking codes from sites that users do not intentionally visit.

Mozilla officials say they have refined that approach in recent months to allow third-party cookies in certain rare cases — for example when a site that a person visits regularly uses a different Web address, which sometimes is done for security purposes.

Mozilla also plans to add new limits on cookies placed by sites users intentionally visit, such as Facebook, to prevent tracking when users sign off and move to other sites. Over the next few months, Mozilla plans to test and further refine its technology before making it available to all users.

To help navigate the complexities of when to allow or disallow tracking, Mozilla has teamed up with Stanford University’s Center for Internet and Society to create a “Cookie Clearinghouse” that will advise the company on how to tweak its settings to protect users. Makers of the Opera Web browser also has joined the Stanford-led initiative.

The clearest losers in Mozilla’s plan will be companies that track users without their knowledge. They will be permitted to request permission to place a cookie in Firefox but users may have little incentive to allow a company they don’t know to access to their personal Web browsing data.

McDonald formerly was co-chair of a two-year-old effort to get the advertising industry, browser makers and privacy advocates to agree on an initiative called “Do Not Track,” which was endorsed by the White House and Federal Trade Commission. It was supposed to give users the ability to block tracking by changing the settings on their browsers — no matter what company made them — but the sprawling working group has struggled to reach consensus.

Mozilla officials and McDonald said that if the “Do Not Track” effort yields an agreement, they may seek to adjust Firefox’s blocking technology and the “Cookie Clearinghouse” to bring them in line with such a deal.

“It’s still not clear if ‘Do No Track’ will do a lot for privacy or only a little,” she said.

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